Typically, a gate valve in which a flange that is continuously formed on the periphery of an opening of a disc attaching port of the valve body (or is formed on the disc attaching port of the valve body) and a flange that is formed on the bonnet are fastened to each other in a watertight state using bolts and nuts is used as a gate valve to be connected to a fluid piping system.
A bonnet removal method including:
the step of attaching a lower housing of a dividable structure to upstream and downstream fluid pipes that are connected to and in communication with a valve body of the gate valve, such that a watertight state is achieved, the dividable structure surrounding approximately the entirety of the gate valve so as to include part of the two fluid pipes;
the step of externally clamp-fixing the flange of the valve body and the flange of the bonnet using a flange clamping device that is provided for the lower housing, thereby maintaining the two flanges in a watertight state;
the step of removing bolts and nuts that fasten the flange of the valve body and the flange of the bonnet to each other;
the step of coupling, to an upper end portion of a stem of the gate valve, a lower end portion of a lower divisional elevation shaft portion out of a plurality of divisional elevation shaft portions that constitute an elevation shaft of an elevation transport device that is to be installed to the upper housing;
the step of coupling and fixing a lower flange of a work open/close valve to a flange of an upper opening portion of the lower housing using bolts and nuts such that a watertight state is achieved;
the step of coupling the upper divisional elevation shaft portion of the elevation transport device installed to the upper housing, to the lower divisional elevation shaft portion coupled to the stem of the gate valve, and coupling and fixing the lower flange of the upper housing to the upper flange of the work open/close valve using bolts and nuts such that a watertight state is achieved;
the step of releasing the clamp fixing of the flange of the valve body and the flange of the bonnet performed by the flange clamping device, thereafter performing an operation to move up the elevation shaft of the elevation transport device, taking out the bonnet provided with the disc and the stem to the inside of a storage space of the upper housing, and performing an operation to close the work open/close valve; and
the step of removing the bonnet taken out to the inside of the storage space of the upper housing
is proposed as a conventional bonnet removal method in which such a typical gate valve is used (see Patent Document 1, for example).